A device is known to detect a travel lane from an image of a road surface (Japanese Patent Unexamined Publication No. 2005-100000). The device disclosed in Japanese Patent Unexamined Publication No. 2005-100000 generates a horizontal edge histogram for a plurality of edge points projected in reverse on coordinates of the road surface. The device then obtains peak positions in the edge histogram and makes the edge points contributing to the respective peak positions into a group, so as to detect lane markers.
However, the number of edge points detected may be decreased on a shoulder of the road since dirt tends to adhere thereto, or the number of edge points detected may be decreased as a distance from the vehicle to the edge points increases. The device disclosed in Japanese Patent Unexamined Publication No. 2005-100000 thus has a problem that lane markers parallel to each other drawn on the travel lane may be estimated in a state of not being parallel to each other.